Ugi 5-center-4-component reaction of α-amino aldehydes and its application in synthesis of 2-oxopiperazines

A synthetic route leading to densely functionalized 2-oxopiperazines is presented. The strategy employs a 5-center-4-component variant of Ugi multicomponent reaction followed by a deprotection/cyclization sequence. N-Boc-α-amino aldehydes were used for the first time as carbonyl components in a key Ugi 5-center-4-component reaction (U-5C-4CR). It is shown that the presented synthetic route can lead to rigid, heterocyclic scaffolds, as demonstrated by the synthesis of tetrahydro-2H-pyrazino[1,2-a]pyrazine-3,6,9(4H)-trione β-turn mimetic and derivatives of 1,6-dioxooctahydropyrrolo[1,2-a]pyrazine and 3,8-dioxohexahydro-3H-oxazolo[3,4-a]pyrazine. Graphical abstract Supplementary Information The online version contains supplementary material available at 10.1007/s11030-023-10760-1.


Introduction
Multicomponent reactions (MCRs) are reactions where more than two starting materials form a single product.Importantly, they proceed with high atom economy and they have propensity to generate molecular diversity by using simple, one-pot procedures, starting from a wide variety of readily available building blocks.These example features make the MCRs particularly attractive for the rapid synthesis of libraries of compounds for drug-discovery purposes [1].
Among the known MCRs, the isocyanide-based Ugi reaction [2] has received much attention in medicinal [3,4] and macrocyclic chemistry [5][6][7], chemical biology and bioconjugation [8][9][10], as well as in natural product synthesis [11,12].Several variants of this condensation have been developed, the Ugi three-(U-3CR) [13,14] and four-component reactions (U-4CR) [15] being most extensively studied (Scheme 1).The first is a reaction of an amine with a carbonyl and an isocyanide, which results in a formation of an α-aminocarboxamide (I), while the latter employs an additional carboxylate component to produce an α-acylaminocarboxamide (II).These two reactions are often followed by the subsequent post-condensation modifications and both have been used as powerful tools for generating diverse scaffolds for drug discovery purposes [16].Another interesting but less studied variant is the Ugi 5-center-4-component reaction (U-5C-4CR) [17], which employs carbonyls, isocyanides, alcohols and α-or β-amino acids 1 3 as bifunctional reagents to provide α, α′-imino dicarboxylic acids (III).Similarly to U-3CR and U-4CR, U-5C-4CR has a proven potential to generate libraries of small-molecular scaffolds.Importantly, depending on the combination of condensation components, all variants of Ugi MCR can deliver C(sp 3 )-rich amino acid derivatives and peptidomimetics [18,19].
The 2-oxopiperazine framework is found in multiple biologically active compounds, such as arenavirus cell entry [20], factor Xa [21] and membrane-associated hepatitis C virus (HCV) Protein NS4B inhibitors [22], as well as in natural products [23,24].Recently, Arora and co-workers have shown that 2-oxopiperazine helix mimetics (OHMs, Fig. 1) can be useful templates for design of protein-protein interaction (PPI) inhibitors [25][26][27].One of our projects utilized a variation of this approach and required access to highly functionalized 2-oxopiperazines (Fig. 1) projecting aromatic amino acid side chains towards their respective hydrophobic pockets localized in the PPI interface.
Similar compounds have previously been synthesized by various strategies [23,[28][29][30][31][32][33], including those based on the U-4CR [30,31], the 'disrupted' Ugi [30] and the Castagnoli-Cushman [31] MCRs.A retrosynthetic analysis showed that the target, functionalized 2-oxopiperazines can be assembled by a short Ugi/deprotection/cyclization sequence based on the U-5C-4CR variant (Fig. 2).The potential advantages of this approach are: low number of reaction steps; high atom economy; operational simplicity of the respective reactions; capability of functionalization of the 2-oxopiperazine scaffold with amino acid side chains; and potential of generating structural diversity at five atoms of the 2-oxopiperazine framework by varying the respective components of the U-5C-4CR, or by postcondensation modifications of secondary amine nitrogen in products for which N-unsubstituted amino acids are used as inputs.Moreover, a proper combination of accessible, enantiopure starting materials may lead to final products with a desired stereochemistry.
The U-5C-4CR step of the proposed synthetic pathway employs the condensation of N-protected α-amino aldehydes with α-amino acids, isocyanides and MeOH.Although various carbonyl compounds have been used in the Ugi MCR, only a few literature reports exist on the condensations of N-protected α-amino aldehydes.Such components were coupled with the respective α-amino esters or amines in the 'classical' 4-center-4-component reaction (U-4C-4CR) [30,34,35] and the similar 4-center-3-component (U-4C-3CR) [36] and 5-center-5-component (U-5C-5CR) [37] variants, whereas no reports exist on their use as condensation partners for α-amino acids in the U-5C-4CR.It is worth noting, that the reported outcomes of the mentioned Ugi MCRs differed significantly, and each time the unique sets of products were formed (Fig. 3).Driven by the need for access to chiral 2-oxopiperazines functionalized with amino acid [28,29] side chains and by the fact that their precursors, the unprecedented U-5C-4CR products, may open access to novel peptidomimetic chemical space, in this paper we investigate the usefulness of the N-protected α-amino aldehydes as carbonyl components in U-5C-4CR.Further, we show that such adducts can be efficiently used in cyclization reactions to the substituted 2-oxopiperazines having five potential diversity points and a defined stereochemistry.Finally, we demonstrate, that the proposed method can deliver useful intermediates for the assembly of rigid, C(sp 3 )-rich, bicyclic scaffolds.

Results and discussion
Several factors influence the outcomes of U-5C-4CR, among them the nature and the degree of bulkiness of the condensation components, the temperature and the presence of the Lewis acid catalyst, which is postulated to facilitate the imine formation/activation during the early steps in reaction course [38][39][40][41].We performed U-5C-4CR using optimized conditions (Table 1), with equimolar amounts of the N-Bocprotected α-amino aldehydes, α-amino acids and isocyanides in MeOH (0.5 M solution), in the presence of Sc(OTf) 3 , at 60 °C.
Using these conditions, we investigated the substrate scope of U-5C-4CR of N-Boc-protected α-amino aldehydes (Scheme 2).First, we tested various amino acids as coupling partners for N-Boc-L-phenylalaninal, tert-butyl isocyanide and MeOH.The products 1a-l were formed with fair to very good yields, glycine adduct 1a being the only notable exception (25%).In most cases, the yields only slightly depended on the steric bulk of the starting amino acids.The steric hindrance introduced by the branched or aromatic amino acid side chains did not affect the yields.Thus, the adducts 1f-i were generally formed with similar yields (52-83%) to those obtained by U-5C-4CRs of less branched or less bulky amino acids 1a-e (25-68%), the aforementioned glycinate adduct 1a being the most notable example.Secondary amino acids have also proven suitable coupling components of U-5C-4CR, as illustrated by the products 1j-l that were obtained with yields comparable to those observed for reactions of the primary amino acids (61-70%).Next, we Fig. 3 The outcomes of the reported Ugi condensations of N-protected α-amino aldehydes tested the reactivity of the example, readily available isocyanides.In general, we observed similar or slightly lower yields of condensation products 1m-o of linear isocyanides, as compared with the yields of their respective analogues 1i and 1g derived from tert-butyl isocyanide (1m: 69% vs. 1i: 83%; 1n: 76% vs. 1i: 83%; 1o: 61% vs. 1g: 80%).
Only the glycine derivative 1p of ethyl isocyanoacetate was formed in a higher yield than its analogue 1a obtained from tert-butyl isocyanide (38% vs. 25%, respectively).Finally, the molecular diversity of U-5C-4CR can be expanded by employing various N-protected α-amino aldehydes, as illustrated by the synthesis of compounds 1q-s from N-Boc-L-alaninal, N-Boc-L-leucinal and N-Boc-L-tryptophanal, respectively.
A new stereocenter is formed in the course of U-5C-4CR and diastereoselectivity is observed when enantiopure chiral substrates are employed as the condensation components.
In the examples 1a-s shown (Scheme 2), at least one chiral Scheme 2 The substrate scope of U-5C-4CR of N-Boc-protected α-amino aldehydes.1 Equiv of each U-5C-4CR component was used.Isolated yields refer to the sum of diastereoisomers.The dr values were estimated by 1 H NMR and refer to the purified products starting material was used, and the observed diastereoselectivities were generally low to good (dr values up to 80:20).In general, we observed highest diastereoinduction for the U-5C-4CRs of branched and secondary L-amino acids.Interestingly, slightly higher dr values were obtained when linear ethyl isocyanoacetate was used instead of bulky tert-butyl isocyanide in the synthesis of glycine adducts 1a and 1p.
Next, we investigated the transformation of the U-5C-4CR products 1 to the target substituted 2-oxopiperazines 2 (Scheme 3).The selected U-5C-4CR adducts, 1d, i-l, n-p, were subjected to Boc cleavage in acidic media, followed by the base-promoted cyclocondensation reaction.This operationally simple procedure gave the respective 2-oxopiperazines, 2d, i-l, n-p, with moderate to good yields (43-78%).In the cyclocondensation step, the reaction times varied from several hours to 9 days.Apart from the fact that the fastest reactions took place in the case of the secondary aromatic amino acids (4 h for 2k and 2l), there was no clear trend between the cyclization times and the structures of the U-5C-4CR adducts 1 used as starting materials.We observed a transesterification reaction of ethyl to methyl ester in the synthesis of 2n.This side reaction can be avoided by replacing MeOH with toluene, as demonstrated by the conversion of 1p to 2-oxopiperazine 2p.We next sought to simplify the U-5C-4CR-based synthesis of 2-oxopiperazines, omitting the chromatographic purification of the U-5C-4CR adducts.These attempts were successful, as shown in the synthesis of compounds 2c and 2t.These 2-oxopiperazines were formed in 31% and 19% yields, respectively, over 3 steps, which was acceptable bearing in mind the ease of the protocol and the structural complexity of the final products.The yield of 2c obtained using this procedure was comparable to the one from the three-step synthesis employing purified Ugi adduct 1c (31% vs 29%, overall).
We obtained the bicyclic bis-lactam 2u using a simplified protocol, which was similar to the one previously applied in the synthesis of 2c and 2t (Scheme 4).The HPLC-MS analysis of the U-5C-4CR of L-glutamic acid showed the formation of a complex mixture of byproducts and intermediates 1u and 1u′, separation of which would be impractical.Therefore, we performed a one-pot esterification of 1u/Boc cleavage by simply heating the post-Ugi reaction mixture with HCl (addition of a 4N solution of HCl in 1,4-dioxane).The subsequent NaHCO 3 workup followed by the cyclocondensation reaction gave 2u in a 30% yield (dr = 73:27), over 3 steps.The obtained bis-lactam 2u is a pyroglutamic acid analogue of the bicyclic proline derivative 2j.Both compounds share a cyclo (Prol-Phe)-like Scheme 3 Deprotection/cyclization of the selected U-5C-4CR adducts 1 to substituted 2-oxopiperazines 2. If not stated otherwise, the diastereoisomeric mixtures of 1 were used as starting materials.Isolated yields refer to the sum of diastereoisomers.dr values were measured by 1 H analyses of purified products.The colors indicate the origin of atoms from U-5C-4CR: green, isocyanide; red, α-amino aldehyde; blue, α-amino acid; pink, alcohol.a Synthesized without purification of the respective U-5C-4CR adducts, yield over 3 steps.b A single diastereoisomer of 2k was isolated.c Compound 2n was synthesized from the single (2R, 3S)-1n diastereoisomer 3-benzyl-perhydropyrrolo[1,2-a]pyrazin-1-one scaffold, which is present in potent thyroliberin antagonists [42].Gratifyingly, no epimerization took place during the cyclocondensation step upon prolonged heating of Boc-deprotected U-5C-4CR adducts in the presence of excess of TEA.In most cases, the diastereoisomers of the 2-oxopiperazines 2 were separable by recrystallization and/or by column chromatography on silica.The obtained compounds are cyclic and rigid, which allowed assignment of the stereochemistry of the particular isomers by NMR.The ROESY experiments performed for the respective separated isomers of 2u indicated that the diastereo induction favored the (R)-configuration on the stereo center created in the course of the U-5C-4CR (Scheme 4).To further support this, we solved and refined a crystal structure of the minor isomer of bis-lactam 2u, which revealed a (3S, 4S, 8aS)-absolute configuration.
In addition to the synthesis of 2-oxopiperazines 2 with five potential diversity points accessible by manipulation of the U-5C-4CR components, we investigated the simple transformations of these compounds that would potentially lead to other rigid, C(sp 3 )-rich heterocycles.First, we performed the 1, 5, 7-triazabicyclo[4.4.0]dec-5-ene (TBD)-triggered intramolecular cyclization of (2R, 3S)-2t to obtain 3, a derivative of a tetrahydro-2H-pyrazino[1,2-a]pyrazine-3, 6, 9(4H)-trione scaffold, which had previously been shown to be a valuable scaffold for β-helical mimetics (Scheme 5) [31].We isolated the major isomer of 3 by column chromatography.In a ROESY experiment, we observed a pronounced nOe between the proton attached to the bridgehead C-9a carbon atom and the protons in the C-1 and C-4 (axial) positions, which clearly suggested the (1S, 9aS)-configuration of this diastereoisomer (Scheme 5).The X-ray crystallography confirmed this assignment.This was in line with a literature report on high levels of epimerization at the C-9a stereocenter triggered upon exposure of compounds similar to 3 to a strong base, which favors a cis-configuration of the protons in C-9a and C-1 positions.Gratifyingly, it was also reported that the tetrahydro-2H-pyrazino[1,2-a] Scheme 4 Synthesis and assignment of absolute configurations of diastereoisomers of 2u.The crystal structure of the minor diastereoisomer (3S, 4S, 8aS)-2u is shown.The protons attached to the ste-reocenters C-3, C-4 and C-8a are in the axial, equatorial and axial positions, respectively, with regard to the 2-oxopiperazine ring of the fused bicyclic system Scheme 5 Synthesis and assignment of relative configurations of diastereoisomers of 3. The crystal structure of the major diastereoisomer (1S, 9aS)-3 is shown.The protons attached to the stereocenters C-1 and C-9a are in the equatorial and axial positions, respectively, with regard to the 2-oxopiperazine ring of the fused bicyclic system pyrazine-3,6,9(4H)-triones of this stereochemistry are capable of effectively mimicking the peptide β-turn [31].
As demonstrated by the synthesis of 2u, the derivatization potential of the 2-oxopiperazines obtained via U-5C-4CR/deprotection/cyclization sequence depends not only on the presence of a reactive group introduced by the isocyanide in the Ugi step (as in the synthesis of 3), but can also result from the reactivity of a side chain of the α-amino acid involved in the condensation.In this context, we sought to explore the usefulness of a hydroxyl and secondary amino groups present in the L-serine derivative 2c.A mild reaction of 2c with 1, 1′-carbonyldiimidazole (CDI) in the presence of TEA afforded compound 4, having a novel, drug-like 3, 8-dioxohexahydro-3H-oxazolo [3,4-a]pyrazine heterobicyclic scaffold (Scheme 6).

Conclusions
In this study, we have shown that N-Boc-α-amino aldehydes efficiently couple with various α-amino acids, isocyanides and MeOH in the course of the U-5C-4CR.The condensations usually gave moderate to high yields, which is satisfactory bearing in mind the operational simplicity of the reaction and the levels of structural complexity of the respective products.We observed moderate diastereoselectivities, with the diastereo induction favoring the (R)configuration of the newly created stereocenter.We have shown, that the obtained U-5C-4CR products may be useful for the construction of libraries of heterocycles that fulfill the drug-likeness criteria.First, we have designed a U-5C-4CR/deprotection/cyclization sequence leading to a library mono-, bi-and tricyclic 2-oxopiperazines functionalized with amino acid side chains.Further, we have shown, that our method may be a complementary approach to the MCRbased synthesis of the tetrahydro-2H-pyrazino[1,2-a]pyrazine-3, 6, 9(4H)-trione β-turn mimetics developed recently [31], which potentially leads to analogs with unique, amino acid-derived substitution patterns.Finally, by synthesizing the tetrahydropyrrolo[1,2-a]pyrazine-1,6(2H, 7H)-dione and the unprecedented 3,8-dioxohexahydro-3H-oxazolo [3, 4-a]pyrazine heterobicyclic, C(sp 3 )-rich scaffolds, we have shown that the convertible amino acid side chains may be useful for further derivatization of 2-oxopiperazines.In conclusion, our results show that the presented method can generate principal components of structural diversity (appendage, functional group, stereochemical and skeletal diversity) [43] within the 2-oxopiperazine framework, in an operationally simple manner, using commonly available reagents.

General procedure for U-5C-4CR
Isocyanide (1 equiv) was added to the mixture of α-amino acid (1 equiv), N-Boc-α-amino aldehyde (1 equiv), Sc(OTf) 3 (0.1 equiv) in MeOH (2 mL per 1 mmol of isocyanide, degassed by passage of Ar gas for 20 min).The mixture was stirred at 60 °C overnight and the solvent was evaporated in vacuo.The residue was purified by CC to give the corresponding iminocarboxylic acids as diastereomeric mixtures (1a-s) that were not separated.The samples of pure diastereoisomers of 1n and 1q were obtained by repeated CC.Due to the dynamic processes (rotamers), line broadening in the 13 C NMR spectra of the U-5C-4CR products 1 is observed.